Fabrication of p-type 2D single-crystalline transistor arrays with Fermi-level-tuned van der Waals semimetal electrodes

High-performance p-type two-dimensional (2D) transistors are fundamental for 2D nanoelectronics. However, the lack of a reliable method for creating high-quality, large-scale p-type 2D semiconductors and a suitable metallization process represents important challenges that need to be addressed for future developments of the field. Here, we report the fabrication of scalable p-type 2D single-crystalline 2H-MoTe2 transistor arrays with Fermi-level-tuned 1T’-phase semimetal contact electrodes. By transforming polycrystalline 1T’-MoTe2 to 2H polymorph via abnormal grain growth, we fabricated 4-inch 2H-MoTe2 wafers with ultra-large single-crystalline domains and spatially-controlled single-crystalline arrays at a low temperature (~500 °C). Furthermore, we demonstrate on-chip transistors by lithographic patterning and layer-by-layer integration of 1T’ semimetals and 2H semiconductors. Work function modulation of 1T’-MoTe2 electrodes was achieved by depositing 3D metal (Au) pads, resulting in minimal contact resistance (~0.7 kΩ·μm) and near-zero Schottky barrier height (~14 meV) of the junction interface, and leading to high on-state current (~7.8 μA/μm) and on/off current ratio (~105) in the 2H-MoTe2 transistors.

: Phase transition was not successful at the side of the sample, where the Te vapor may have tended to escape from the enclosed to opened regions, resulting in the formation of the d2H phase. Furthermore, at a much higher growth temperature (T > 800°C), the 2H MoTe2 crystal exhibits thermal instability, which leads to the formation of a mixed 1T'-d2H structure in our sample ( Supplementary  Fig. 2n). This may be attributed to the faster evaporation of Te from the NixTey precursor at 800 °C, resulting in a Te-deficient condition that induces the formation of the 1T' structure rather than the 2H phase. Supplementary Fig. 3. Comparison of the reported growth time to obtain the largest film dimensions in 2H-MoTe2 thin film. Each symbol represents the reported parameters in ref [2][3][4][5][6][7][8] .
: Our growth method for 2H-MoTe2 has advantages over previous studies regarding processability with low energy (Supplementary Table 1). The growth time required for MoTe2 across the 4-inch (~101 mm) wafer was moderate (~2 h) compared with a recent report (~72 h) 6 . The growth temperature can be decreased to 500 °C by seeding the 2H crystal, which is compatible for the direct integration process of CMOS back-end-of-line. Although MOCVD may reduce the growth temperature and time for manufacturing tri-layer MoTe2 4 , the resultant film was polycrystalline and exhibited inferior electrical properties; the importance of achieving high-quality crystals for a transistor is indicated in Figs. 4h, k (and Supplementary Fig. 21).
We assumed that our gas-confined growth mode enabled the production of high-quality MoTe2 on a 4-inch wafer scale. The gas-confined reactor provided a high Te flux favorable for phase transition, whereas its vertical flow assisted the homogenous nucleation across the large scale. In contrast, the lateral delivery of Te flow can result in inhomogeneous random nucleation and Te deficiency during growth. However, most reports adopted the horizontal flow system, except for Refs. 4,6 in Supplementary Table 1. Supplementary Fig. 4. XPS characterizations for 2H-and 1T'-phase MoTe2. XPS spectra of the Mo 3d core level captured at the 2H-(blue) and 1T'-phase crystals (red) contained at the thin films grown at T = 700 °C for t = 30 min and T = 500 °C for t = 10 min, respectively. For comparison, the spectra of the as-grown MoTe2 tri-layer film are displayed. The difference of Raman mode 9 (that is, A1g, Eg, and A1g in the bilayer compared to A' E' and A'' in the tri-layers), peak locations, and breath modes 10  : To synthesize the 2H-MoTe2 bilayer, we used MoOx as a metal precursor instead of Mo, which allowed the atomic smoothness without void formation by decreasing the volume expansion of the precursor during the tellurization. Successful growth was suggested by the characteristic Raman modes of A1g, Eg, and A1g ( Supplementary Fig. 6b, c) 9 and the breathing modes of few-layer structures depending on thickness 10 (arrows in Supplementary Fig. 6c). An AFM in Supplementary Fig. 6e displayed its atomically smooth surface with a thickness of ~1.6 nm and no visible micro-voids (which contradicts the growth of ~3-nm-thick film in Ref. 6 ). The optical absorption spectrum of the film grown on quartz indicated that the band-to-band excitonic transition occurred at ~1.05 eV for the 2H-MoTe2 bilayer ( Supplementary Fig. 6d). An atomic-resolution STEM in Supplementary Fig. 6f suggested the production of MoTe2 with a 2H structure. The thin film consisted of a bilayer across a large area with small monolayer domains (~2 nm in lateral dimension), but no visible holes were detected. XPS results in Supplementary Figs. 6g, h revealed the Te-Mo and Mo-Te bindings in the Te 3d and Mo 3d scans, respectively. The absence of oxide features and estimated at.% of Te/Mo (~2.01) suggested the high crystallinity of the thin film. Notably, bilayer MoTe2 is the thinnest film that can be obtained using the CVD mode on a millimeter scale (Supplementary Table 1 11 showing the potential phase-change performance. : The measured WF of the 1T' structure was 4.51 eV, whereas WF of the 2H phase was 4.44 eV. With regards to the valance band cutoff edge ( Supplementary Fig. 7a), the binding energy (Eb) of the 2H phase was 0.57 eV, corresponding to its energy level of valence band maximum (EF -EVBM). In contrast, the 1T' structure showed a sharp increase in the intensity at Eb = 0 eV owing to the filled energy levels of the metal.
To gain a better understanding of the band structure of 2H-MoTe2, the ε was measured ( Supplementary Fig. 7c). The peaks that signifying each transition along the Brillouin zone were labeled in the spectra 12,13 . For instance, the lowest direct optical excitonic transitions at the K-point (Aand B-exciton transitions) 13,14 were discovered to be at ~1.04 eV and ~1.36 eV, implying that the valance band splitting by spin-orbit coupling effect was at ~318 meV. Additionally, the Eg was calculated by following the Tauc's equation 15 ; ν 2 ε = (hν -Eg) 2 where ν = 2π/λ is the angular frequency of the incident radiation, and λ is the wavelength. As demonstrated in the inset of Supplementary Fig.  7c, the x-intersection of the Tauc plot (1/λ vs. ε 1/2 /λ) indicates the 1/Eg, and the Eg for our 2H MoTe2 was extracted to be ~0.89 eV, which is in good agreement with the values obtained from the literature 12,14 . Conversely, the 1T'-phase MoTe2 did not show any characteristic feature in the ε-E plot, indicating it is a gap-less material ( Supplementary Fig. 7d). Thus, the appropriate phase transition of MoTe2 by our method makes it possible to engineer the band structure effectively, as depicted in Fig.  1i, satisfying the various requirements for electronic components.
It should be noted that the bandgap of our MoTe2 thin film (~0.89 eV) is smaller than other group-VI TMDs, which enables MoTe2 to absorb light from a broader spectral range, including the visible (VIS) and near-infrared (NIR) ranges, showing a higher absorption coefficient (α > 10 4 cm -1 ) (Fig. 1j). Furthermore, the high α values in our as-grown MoTe2 are close to those of mechanically exfoliated single crystals 16 , indicating high crystalline quality (red curves in Fig. 1j). The α value is even higher than that of bulk semiconductors with similar bandgaps, such as Si 17 or Ge 18 , which are conventionally used in photodetectors. The large α values are the result of the lowest direct transition in MoTe2, which has an energy close to the indirect bandgap; this is not the case for Si and Ge 19 . This suggests that MoTe2 thin film has great potential for use in optical communication devices, including saturable absorbers 20 , modulators 21 , and photodetectors 22 .
In particular, we believe that MoTe2 FETs can be used as high-performance photodetectors to cover the NIR range, which cannot be achieved with other 2D TMDs with larger bandgaps. Under NIR illumination, the photoresponsivity of transistors can be enhanced by the photo-gating effect. In addition, the asymmetric contact barriers formed using different drain-source electrodes can further enhance the rectification ratio 23 . Our Fermi-level-tuned 1T'-MoTe2 can serve as an efficient vdW contact for hole transport, which promises a fast photoresponse owing to fewer charge traps at the MSJ interface. : TEM analysis in (a) indicates that the polymorphic MoTe2 crystals have a similar orientation, which can be the most favorable alignment for the grain growth of 1T' polycrystals to transform to a large 2H-phase single crystalline domain (i.e., abnormal grain growth). Conversely, many of the in-plane polymorphic interfaces demonstrated random orientations (b, c), which indicates that the Te-rich environment during high-temperature CVD may enable the growth of grains by overcoming the crystallographic randomness. : The Williamson-Hall approach takes into account the broadening of the peaks as a function of a diffraction angle (2θ), which considers the combined impact of the size (βD) and strain-driven widening (βS) as follows: Hence, the extracted slopes of the Williamson-Hall plot (4sinθ-βcosθ) in Supplementary Fig. 10c will be the ε values. : The color uniformity of the as-grown MoTe2 confirmed that our method was suitable for obtaining uniform films, as shown in Case A ( Supplementary Fig. 11b). Furthermore, in Case A, circular-shaped 2H domains of approximately 30-100 μm with a coverage of ~31.6% (over 1 x 1 cm 2 substrate) were evenly distributed inside the 1T' thin film, indicating abnormal grain growth ( Supplementary Fig. 11c). In contrast, in Case C, grain growth in the 2H phase larger than several micrometers did not occur ( Supplementary Fig. 11i). The 1T' and 2H phases in the thin film were intermixed, and their densities varied across the film, presumably because of the non-uniform Te flux (as seen by the ambiguous optical contrast in Supplementary Figs. 11h, i). (c-f) OM images of the sample heated at T = 500 °C for t = 10 min using different carrier gases and Te sources: (c) without a Te source and Ar/H2 mixed gas; (d) with 0.1 g of Te powder placed next to the sample and Ar/H2 mixed gas; (e) using the NixTey stack for Te-gas confinement and Ar/H2 mixed gas; and (f) using the NixTey stack and only Ar gas. The insets in (e, f) are zoomed-in OM images showing the interfaces. (g-i) STEM-EDS analysis of the seed growth: (g) low-magnified HAADF-STEM image of the structure; (h) corresponding EDS mapping images for Mo (left) and Te (right) atoms; and (i) EDS spectra for the different regions marked in (f). The stoichiometry of the structure (i.e., at.%(Te/Mo)) averaged for four different sampling regions was ~2.13 ± 0.06, 2.10 ± 0.12, and 2.02 ± 0.10 for the 2H-seed, tr-2H, and 1T' region, respectively. The sample for STEM-EDS analysis was produced by heat treatment at T = 500 °C followed by rapid cooling, i.e., furnace cover opening at T = 500 °C. Rapid cooling allows the Te adatoms to remain on the surface because there is insufficient time for the absorbed Te to be desorbed by thermal energy. : In this study, the initial seed layers were made of mechanically exfoliated single-crystalline flakes. The "bulk" MoTe2 mother crystal (obtained from HQ graphene) had a single crystalline property over a large area (> ~5 mm). Different flakes transferred on the tape had the same crystal orientation when adhesive tape was simultaneously applied and removed from the crystal. This tape/flake sample was then transferred onto a preformed 1T'-MoTe2 film (with a thickness of 20 nm), followed by seed growth and conversion to the 2H phase at 500°C. The low-temperature growth at 500 °C allowed the suppression of random 2H nucleation during the seed growth process. The resulting fully grown 2H film was single-crystal in nature along the unidirectional crystalline orientation of the exfoliated seeds (at least across an area of ~2 × 2 mm 2 , as shown in Supplementary Figs. 13a-c). The seeded-grown 2H-flakes/film structure could be patterned using photolithography for subsequent seed growth to synthesize unidirectionally oriented single crystals, as shown in the OM image ( Supplementary Fig. 13a). Inverse pole figure mapping ( Supplementary Fig. 13b) and EBSD ( Supplementary Fig. 13c) patterns confirmed that the conformally formed patterns were single crystals with (0001) texture and the same alignment or thin films with a small-angle GB (with a standard deviation of ~1.19°). Moreover, TEM analysis showed that the newly grown "tr. At first, the synthesis was conducted at the different growth T and t for a specific phase; for example, T = 700 °C, t = 60 min, and T = 500 °C, t = 30 min for the 2H and 1T' structure, respectively. To achieve the Au/1T'-MoTe2 patterns, the arrays of Au layers (~40 nm) were deposited using standard lithography and an e-beam evaporator, and then the reactive ion etching (RIE) process removed the exposed 1T'-MoTe2, except for the underlying structure below the Au patterns. Next, the dry transfer process using the thermal release tape and the polymeric supporting layer of PMMA allowed the achievement of the Au/1T'-MoTe2/2H-MoTe2 junction. The RIE procedure was followed for the definition of channel width for FETs. The inset on the right corner shows a top-view OM image of a fabricated FET with a 2H-MoTe2 channel. : Au/1T'-MoTe2 and Ag/1T'-MoTe2 sample peaks exhibit shifts towards lower binding energy compared to those of the pristine 1T'-MoTe2, which is unrelated to any stoichiometric change ( Supplementary Fig. 15f) and the chemical interaction between 3D metals and Te. Instead, we believe that the high-carrier-density 3D metal deposition on the 1T'-structured semimetal impacts its electronic properties, shifting the Fermi level by charge transport (i.e., doping). For instance, the UPS-driven WF of the Au/1T'-MoTe2 was ~5.0 eV, showing the deviation from the as-grown layer with a WF of ~4.45 eV (Fig. 5b).  34,35 or oxidation-related doping 41,42 methods or used contact metals (e.g., graphene (gr.) 37,38 , the transferred (tr.) metals [43][44][45][46] , and the cleanly deposited metals with vdW gap (vdW metals) 23,40 ) are noted (see Supplementary Table 4 for more comparisons). Most of the WSe2 transistors reported here were based on the irregularly synthesized or mechanically exfoliated flakes, except MOCVD 39 and this study.
: The challenge in preparing a superior p-type device is contradictory to the technological maturity for the construction of n-type 2D transistors for MoS2, particularly compared to the cases using low-melting-point metals such as Bi 49 and In 40 (note that Ion of MoS2 approached 1,135 μA·μm -1 in Ref. 49 ). For example, WSe2 is a promising candidate as a unipolar p-type channel owing to its relatively high valence band edge compared with MoS2 and WS2. However, most CVD-grown WSe2 still exhibited the Ion values of 0.02-3.30 μA·μm -1 under the Vds of -1 V 34-39 even after its doping 34,35 or employing 2D metal contact electrodes (e.g., graphene) 37,38 , which is more than two times lower than that of our synthetic MoTe2 polymorphic transistor (~7.8 ± 1.4 μA·μm -1 ) (see Supplementary Fig.  19a and Supplementary Table 4 for the comparisons). Furthermore, the calculated Rc in CVD-WSe2 (16.3-10 5 kΩ•μm) 23,34,37,38 was higher than that obtained in this study (Supplementary Table 4). Note that the similar Ion of ~7.6 μA· μm -1 in a CVD-WSe2 flake could be achieved by the ultraclean Pt contact 23 , but the deposition process required a longer period (~4 h) to reduce the irradiation energy. Although one study proved that the synthetic vertical heterostructure of VSe2/WSe2 allowed the large Ion of ~1,580 μA· μm -1 in an ultrashort channel (~20 nm) 50 , the process is still embryonic in terms of device manufacturing in any desired channel dimension using standard lithographic techniques. Nearly all CVD-WSe2 transistors were constructed based on tiny irregular flakes rather than a wafer-scale film 23,[34][35][36][37][38]50 (owing to the limitation of scalability), raising the question of reproducibility and deviceto-device variations.
The Ion of our p-type MoTe2 transistor was also comparable to or even higher than those reported for high-performance devices manufactured for mechanically exfoliated WSe2 23,40-48 ( Supplementary Fig. 19b and Supplementary Table 4). Compared to the thicker WSe2 transistors (5-9 layers 41,42,44,47,48 or unidentified 23,40,45,46 ), the superior Ion reported in our study indicates that MoTe2 can also be an outstanding candidate for p-type 2D transistors when the high-quality channel and defectfree contact are combined. In addition, the valence band edge of the MoTe2 higher than that of WSe2 51 allows more unipolar p-type transport in a transistor, which has the advantage of achieving lower power consumption and faster operation in the CMOS inverter. : MoTe2 can behave as a better p-type channel than other group-VI 2D TMDs in CMOS, given its band structure. Typical group-VI 2D TMDs other than 2H-MoTe2 present a challenge in suppressing electron transport owing to their EVBM edge located at > 5.0 eV (Supplementary Fig. 20a). This results in a high SBH for holes (Φhole at VFB) in these 2D TMDs, which limits the hole conductivity ( Supplementary Fig. 20b). In contrast, MoTe2 has a lower SBH owing to its higher location of EVBM, making its hole conductivity higher (Supplementary Fig. 20b). Notably, the ratio between Φhole and Φelectron of MoTe2 is also the smallest among the group-VI TMDs (Supplementary Fig. 20c). Consequently, MoTe2-based 2D FETs tends to exhibit p-type unipolarity (instead of ambipolarity), promising a low power-delay product per bit in CMOS owing to the smaller off-state currents in ptype MOS and greater efficiency in high-low and low-high transitions.

Supplementary Fig. 21. Effect of oxidation and GBs on the electrical transport of 2H-MoTe2
FETs with 3D metal contact electrodes. (a, b) Oxidation-induced p-type doping of a 2H-MoTe2 FET with Pt contact electrodes. (a) Transfer curve (Ids-Vg) of the FET depending on the oxidation time (e.g., 1, 10, and 30 min). Here, the oxidation process 53 was conducted using a commercial ultraviolet (UV)-O3 cleaning system (Ahtech, AC-3). (b) Evolution of the on/off ratio and µ as a function of the oxidation time (t). Oxidation was observed to degrade switching behavior by decreasing the on/off ratio as the Ioff gets higher. Numerous studies on 2H-MoTe2-based FETs show a small on/off ratio (< 10 3 ) despite its high µ and low sheet resistance, which could probably be affected by the oxidation, as MoTe2 is vulnerable to oxidation more than other semiconducting TMDs. : The direct synthesis method for fabricating 2D/2D polymorphic heterojunction resulted in a poor FET performance in comparison to the transfer process we suggested in the main text ( Supplementary  Fig. 14). Contact properties could be degraded due to the amorphous structure ( Supplementary Fig.  22b) and non-stoichiometric MoTe2 (Supplementary Fig. 17c). We believe that the direct deposition of Mo on top of as-grown 2H-MoTe2 using DC sputtering could produce some deformed phases, as we observed new peaks for 1T'-like structures in the XPS of the Mo/2H-MoTe2 (Supplementary Figs.  17a, b). Nevertheless, there are some viable options for producing vdW contacts using the synthesis method instead of transfer, where the process includes the vertical growth of TMDs without any interfacial defects 50,[54][55][56] . We believe that the low-energy deposition of MoOx or MoI2 as precursors for 1T'-MoTe2 is a possible solution for preparing ultraclean vdW contact electrodes in a synthetic manner devoid of performance degradations 50,54 . The use of thermally degradable buffer layers 55,56 is another option for realizing vdW 1T' MoTe2 contacts. : The 1T'-MoTe2 thin film was characterized using the four-point probe and TLM ( Supplementary Fig. 23). The method offered significant control over the H (≈3.5-13 nm) and enabled the systematic characterization of the H-dependent Rsh for a large-area thin film (> 1 × 1 cm 2 ) via fourprobe measurement (Supplementary Fig. 23a). The Rsh values were comparable to those of singlecrystalline 1T' MoTe2 57 , indicating that the crystals are of high quality. The Rsh was increased as the layer got thinner, owing to the enhanced carrier scattering. As a result of the position-controlled growth method of MoTe2 (see the method), the width-defined layer with H of ~5.3 nm was simply contacted to the Ti/Au TLM patterns via conventional photolithography and deposition approaches ( Supplementary Fig. 23b-inset). Ids-Vds curve illustrates the linear relations with a channel length dependence, which is typical evidence for ohmic contacted TLM devices (Supplementary Fig. 23b). Rsh (~7.0 kΩ·sq -1 , which is comparable to the TLM-extracted values for mechanically exfoliated single crystals 58,59 ) and Rc (≈ 0.51 kΩ·cm) values of the 1T'-MoTe2 device were determined using the linear fit to the L-dependent R (Supplementary Fig. 23c). Notably, the Rc was at the low end of the reported 3D metal/2D metal systems values 60 . Additionally, the 1T'-MoTe2 showed a negligible dependence on the Vg (Supplementary Fig. 23d, e), which is a characteristic feature of a gap-less electrical conductor. : We utilized the TLM approach to determine the on-state Rsh, estimated to be ~44.3 ± 2.3 kΩ/sq at Vg = -100 V (as observed from the slopes of TLM curves in Fig. 4i and Supplementary Fig. 24a). The on-state Rsh value represents a "material-dependent" quantity and does not incorporate contributions from device dimensions and contact resistance. Our analysis of on-state Rsh of 2H-MoTe2 indicates that our synthesis method results in the lowest Rsh (44.3 ± 2.3 kΩ/sq), the highest Ion/Ioff ratio (> 2.9 × 10 5 ), and the smallest layer numbers (~6 layers), compared to those of TLM-analyzed CVDgrown MoTe2 FETs in previous studies 3,5,24,25,27 (Supplementary Fig. 24c, d).
Furthermore, the relationship between the on-state Rsh (~44.3 ± 2.3 kΩ/sq) obtained through TLM and the Vg-induced carrier concentration (n2D ~7 × 10 12 cm -2 ) permits us to determine the "intrinsic" field-effect mobility ( ) of our MoTe2 using the following relationship 65-67 : The calculated value of is ~20.2 ± 1.1 cm 2 V -1 s -1 , representing an inherent channel property that is unaffected by contact resistance. Notably, this value closely matches the averaged two-terminal fieldeffect mobility (μh ≈21.0 ± 3.3 cm 2 V -1 s -1 ; inset of Supplementary Fig. 14e), indicating that our 2D semimetal contact electrodes have a minimal impact on μh. Moreover, the TLM-extracted value in our study (~20.2 ± 1.1 cm 2 V -1 s -1 ) surpasses the calculated values for CVD-grown MoTe2 in previous studies 3,5,24,25,27 (Supplementary Fig. 24e; from the reports 3,5,24,25,27 , the on-state Rsh value is extracted from the slopes of TLM plot, and n2D is obtained using a parallel capacitance model as n2D = Cox(Vg -Vth)/q.). Given that all the compared FETs in Refs. 3,5,24,25,27 utilize a bottom-gate SiO2 dielectric layer, the or n2D values are primarily influenced by material properties such as defect density and doping capacity, rather than the device configuration or dielectric interface. : The gray lines for a power law (μFE ∝ T -γ where γ = 1.69) in Supplementary Fig. 25e indicate the ideal phonon scattering model from the theoretical calculation 68 . For our 2H-MoTe2 with vdW Au/1T' contact, the new damping factor, γ, with T > 218 K was positive (0.92 ± 0.24 (mean ± standard error)), indicating that the transport was still limited by phonon scattering 69 . The slight deviation from the ideal value (~1.69) could arise due to the T-dependence of the effective Schottky barrier height 69,70 and/or interplay between homopolar phonon mode quenching and charge-impurity scattering 71 . In contrast, the 2H-MoTe2 FETs with 3D metal contacts (i.e., Pt and Ti) exhibited a decrease in μFE as T decreased, indicating that transport was constrained by the contact resistance rather than phonon scattering.  Fig. 5f), it is evident that the pristine vdW contact effectively induced the MSJ with a small SBH for holes by following the Schottky-Mott rule. In contrast, the high-energy deposition of Au stimulates the formation of the gap state near the conduction band 43,72 , resulting in the n-type transport behavior. (a, c, e) Measured (circles) and fitted (lines) ψ for the SiO2/Si substrate, 2H-MoTe2, and 1T'-MoTe2 at incidence angles 65° (red), 70° (blue), and 75° (green), respectively, (b, d, f) Measured (circles) and fitted (lines) Δ for the SiO2/Si substrate, 2H-MoTe2, and 1T'-MoTe2 at incidence angles 65° (red), 70° (blue), and 75° (green), respectively.